1. Field of the Invention
The present invention relates to a thermosensitive recording material for use in the fields of printers for computer output or calculator, recorders for medical instrumentation, low-speed or high speed printers, automatic ticket machines, thermosensitive copying, handy terminals, and labels such as POS system.
2. Description of the Related Art
Conventionally, various proposals have been made in a recording material having a thermosensitive recording layer, which contains as main components a colorless or pale color leuco dye and a color developer for coloring the leuco dye, disposed on a base of paper, synthesized paper, plastic film or the like, and which uses a coloring reaction initiated by applying heat, pressure, or the like to the thermosensitive recording layer. This type of the thermosensitive recording material has advantages such that complicated processes such as developing, fixing, and the like are not required, recording can be performed using a relatively simple device within a short period of time, less noise is made, and the required cost is low. Because of these advantages, such thermosensitive recording material is widely used. For example, it is used for copying books and documents, and is used as a recording material of electronic computers, facsimiles, ticket venders, label printers, recorders, and handy terminals.
As the thermosensitive recording material, those coloring in high density at high speed, and providing images and back ground of high fastness are desired.
Recently, many thermosensitive recording materials have been used in the field requiring reliability of recorded images, such as for labels and receipts. Accordingly, a thermosensitive recording material having high storage stability against water and acidic compounds contained in foods, or a plasticizer or oil and fats contained in an organic polymer material used in packaging is desired.
Moreover, in recent years, printing has been often performed by a printer of low torque, because of the trends for energy saving, or downsizing so as to use as a portable handy type of the printer. In this case, a thermosensitive recording material has a low head matching property, printing cannot be carried out in the low temperature and low humidity environment as the thermal head and the thermosensitive recording material stick to each other. Therefore, a high matching property between a thermal head and a surface of the thermosensitive recording material is required.
Conventionally, the aforementioned problems have been attempted to solve by adding a wax agent (i.e. a lubricant) to a protective layer disposed on the thermosensitive recording layer.
Use of zinc stearate and calcium stearate as the wax agent improves the head matching property, but calcium stearate does not provide a sufficient lubricating effect. Moreover, though zinc stearate gives a sufficient lubricating effect, it reduces the plasticizer resistance of the thermosensitive recording material. Furthermore, zinc, which is a main substance of zinc stearate, is regarded as a compound which possibly adversely affect water environment, and use of such material is not very suitable in recent years (see, for example, Japanese Patent (JP-B) No. 2812040).
By using montan wax (see, for example, JP-B No. 3611231) or carnauba wax (Japanese Patent Application Laid-Open (JP-A) No. 2000-355165) as the wax agent, the head-matching property is improved. However, as both wax use natural materials as a main substance, there is a possibility that a supply thereof is unstable depending on the change in the natural environment.
Moreover, there has been proposed a technique in which a lubricant having a melting point of 75° C. or lower is added to a protective layer (see, for example, JP-B No. 3573833). However, when the lubricant having a low melting point, such as 75° C. or lower is used in a protective layer, and a protective layer composite liquid is coated by a coating device and dried by a drier, the lubricant melts at the time of drying and then solidified again after the drying. In such case, the original lubricating function cannot be performed, and it is not suitable for mass-production.
Furthermore, more wax (i.e. a lubricant) has been used to solve sticking. The wax is formed of various raw materials, and various wax such as vegetable wax, animal wax, mineral wax, petroleum wax, and synthesis wax formed by chemical synthesis has been used.
Examples of the vegetable wax include carnauba wax, candelilla wax, rice wax, and castor oil. Examples of the animal wax include bees wax, and Chinese insect wax. Examples of the mineral wax include montan wax. Examples of the petroleum wax include paraffin wax, and microcrystalline wax. Examples of the synthesis wax include hydrocarbon-based synthesis wax such as polyethylene wax, and Fischer-Tropsch wax, and other synthesis wax such as higher fatty acid ester, fatty acid amide, ketones and amindes. Various wax has been developed as listed above.
As raw materials of the wax, other than the case of the synthesis wax, various materials present in nature have been used. These raw materials have a difficulty in stable supply due to the changes in the natural environment, or depletion of raw materials.
There has been a proposal such that a non-oxidized polyethylene wax is used as the wax (see JP-B No. 2636895). However, even with this non-oxidized polyethylene wax, a sufficient lubricating function cannot be attained.
Moreover, there has been a proposal such that oxidized polyethylene wax is used as the wax, and carboxy-modified polyvinyl alcohol is used as a resin of a protective layer, and a polyacrylamide epichlorohydrin-based crosslinking agent is used as a crosslinking agent for the protective layer (see, for example, JP-B No. 3520648). However, in this technique, sufficient conveying performance cannot be attained in the high temperature and high humidity environment. Therefore, desirable functions cannot be attained.